Abstract
Polymer based microfiltration and ultrafiltration membranes are commonly manufactured using methods that take advantage of the phase-inversion phenomenon essential in creating porous structures. To facilitate phase-inversion, organic solvents are fundamental ingredients in manufacturing porous polymer membranes. Commonly used organic solvents such as N-methyl-2-pyrrolidone (NMP), Dimethylacetamide (DMAc), Dimethylformamide (DMF), Dimethyl sulfoxide (DMSO), Tetrahydrofuran (THF), etc. are classified as hazardous materials by most health and environmental agencies. Exposure to organic solvents is associated with liver and lung diseases in humans and animals. Their improper disposal can pose a significant threat to the environment. The development of alternative membrane manufacturing methods that do not use organic solvents is a need of the hour since an eventual prohibition of some of them is foreseen. Like porous polymer membranes, another class of materials called polymer foams are defined by their porous nature. However, most polymer foams’ production methods do not involve any organic solvents. Therefore, the implementation polymer foams as polymer membranes offers the potential to eliminate organic solvents, making the membrane manufacturing process more sustainable, safe, and environmentally friendlier than the current state-of-the-art membrane manufacturing methods. Although polymer foams are used in some filtration applications such as air filters that restrict macroscopic particle sizes, they cannot be implemented as membranes for microscopic separation applications such as ultrafiltration due to the unavailability of open-celled nanocellular foams. Therefore, based on the abundant state of the art available in the field of polymer membranes and polymer foams, this work aims to develop open-celled nanocellular polymer foams capable of ultrafiltration. Development of methods that deliver such foams is pursued, and the creation of foam-based ultrafiltration membranes is aimed, thus eliminating organic solvent usage. Polyethersulfone (PESU) was selected as the base material to produce polymer foams owing to its prominent use in ultrafiltration (UF) membranes. A blend of PESU and poly(N-vinylpyrrolidone) (PVP) was developed and characterized for batch foaming. This method was found to yield nanocellular foams. However, these foams were not directly usable for UF applications due to the presence of a non-foamed skin layer. The non-foamed skin layer was eliminated using an innovative sandwich sample method and aqueous sodium hypochlorite (NaOCl) solution treatment. The resultant foam was found to exhibit retention performance that was comparable to state-of-the-art UF membranes, while the water flux required significant improvements. In order to investigate the potential of a similar blend combination i.e. PESU along with a water-soluble polymer for large-scale production of foams, foam extrusion was pursued, and PESU/poly(ethylene glycol) (PEG) blends were investigated. Unlike the PESU/PVP blend, manufactured through melt-state compounding, the PESU/PEG blend was developed material absorption i.e. by allowing liquid PEG to absorb within PESU taking advantage of the porous structure of PESU flakes and was directly used in foam extrusion. The blend composition was optimized to produce continuous microcellular open-celled foams when CO2 and H2O were utilized as foaming agents at specific process settings. Additionally, adding PEG resulted in a processing temperature 120 – 150 °C lower than that of PESU (320 °C – 350 °C). Using an annular slit nozzle, extruded hollow fibers with open-celled foam structure with an average cell size of 5 µm were produced. The same PESU/PEG blend, when extruded without any foaming agents, resulted in the formation of uniformly distributed closed cell pores with an average pore size of 500 nm throughout the extrudate. However, the extrudate did not maintain the hollow fiber shape due to low melt elasticity at the nozzle. Also, the absence of open porosity posed a significant challenge in any utilization for permeation. In order to address these limitations, a ternary blend PESU/PEG/PVP was developed using the same material absorption method as PESU/PEG. The extrusion of this blend without using any foaming agents resulted in an increased melt elasticity, thus retaining the nozzle's hollow fiber geometry. Also, the ternary blend's extruded hollow fiber showed a higher porosity than the PESU/PEG blend. Subsequently, through optimization of the processing parameters, the porosity was increased. After post-treatment of the hollow fibers with aqueous NaOCl, large portions of PEG and PVP were dissolved, further increasing porosity and making the fibers partially open cellular and permeable. This enabled the functionalization of these hollow fibers as membranes. The different miscibilities of PVP and PEG with PESU resulted in different dissolution mechanisms, combined with the surface evaporation of PEG from the extruded fibers synergistically, resulting in different surface and internal porosities. As a result, the separation layer on the outer surface of these extruded hollow fiber membranes was found to have a pore size of approximately 100 nm and an internal open pore size of < 1 µm. In addition, filtration performance and water flux comparable to state-of-the-art ultrafiltration (UF) membranes were observed. This study found polymer foaming as a viable alternative to traditional polymer membrane production methods that rely on organic solvents. Flat sheet and hollow fiber foam-based membranes were produced using binary and ternary blends of PESU, PVP, and PEG. The characteristics and performance were found to be comparable to those produced using production methods that involve the use of organic solvents. However, further optimization and improvements are needed to fully realize this method's potential. Nevertheless, this research opens up new horizons and fields of research in developing foam-based ultrafiltration membranes that are more sustainable and safe.
